Acoustic telemetry is a method of communication in the well drilling and production industry. In a typical drilling environment, acoustic carrier waves from an acoustic telemetry device are modulated in order to carry information via the drillpipe to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed at the surface.
The theory of acoustic telemetry as applied to communication along drillstrings has a long history, and a comprehensive theoretical understanding was eventually achieved and backed up by accurate measurements (D. S. Drumheller, Acoustical Properties Of Drill Strings, J. Acoustical Society of America, 85: 1048-1064, 1989). It is now generally recognized that the nearly regular periodic structure of drillpipe imposes a passband/stopband structure on the frequency response, similar to that of a comb filter. Dispersion, phase non-linearity and frequency-dependent attenuation make drillpipe a challenging medium for telemetry, which situation is made even more challenging by the significant surface and downhole noise generally experienced.
The design of acoustic systems for static production wells has been reasonably successful, as each system can be modified within economic constraints to suit these relatively long-lived applications. The application of acoustic telemetry in the plethora of individually differing real-time drilling situations, however, is much less successful. This is primarily due to the increased noise due to drilling, and the problem of unwanted acoustic wave reflections associated with downhole components, such as the bottom-hole assembly (or “BHA”), typically attached to the end of the drillstring, which reflections can interfere with the desired acoustic telemetry signal. The problem of communication through drillpipe is further complicated by the fact that drillpipe has heavier tool joints than production tubing, resulting in broader stopbands; this entails relatively less available acoustic passband spectrum, making the problems of noise and signal distortion more severe.
To make the situation even more challenging, BHA components are normally designed without any regard to acoustic telemetry applications, enhancing the risk of unwanted and possibly deleterious reflections caused primarily by the BHA components.
When exploring for oil or gas, or in coal mine drilling applications, an acoustic transmitter is preferably placed near the BHA, typically near the drill bit where the transmitter can gather certain drilling and formation data, process this data, and then convert the data into a signal to be broadcast to an appropriate receiving and decoding station. In some systems, the transmitter is designed to produce elastic extensional stress waves that propagate through the drillstring to the surface, where the waves are detected by sensors, such as accelerometers, attached to the drill string or associated drilling rig equipment. These waves carry information of value to the drillers and others who are responsible for steering the well. There are several ways in which extensional waves may be produced, but for exemplary purposes the following discussion shall concentrate on a transducer comprising a stack of piezoelectric discs (the ‘stack’), arranged physically in series, that are constrained between two metal shoulders disposed on a mandrel, protected by a cover, the stack being energised by the application of a high voltage. As this high voltage is applied it causes the stack to either increase or decrease its axial length, and this is transferred to the mandrel and cover. Elastic deformation of the mandrel and cover due to periodic changes in the applied voltage causes extensional waves to propagate away from the two faces of the stack.
The periodic changes in the applied voltage have a repetition rate that matches one of the passband filter effects of typical drillpipe (A. Bedford and D. S. Drumheller, Introduction to Elastic Wave Propagation, John Wiley & Sons, Chichester, 1994). A simple way to apply a periodic high voltage to a stack is to utilize a transformer whose secondary winding is connected to the stack, and whose primary winding is attached to a switching unit and a power source, such as a battery. Although there are other ways of achieving a switched high voltage across the stack, this example shall be employed in the following for illustrative purposes. The stack's major electrical characteristic is as a capacitor, while the transformer appears most significantly as an inductance. In order that the transmitter system is run efficiently it is helpful to make the practical transformer/stack combination (i.e. tank circuit) resonant with a resonance quality factor (Q) of the order 4 to 10. It will be evident that the most efficient utilization of such a resonant circuit is to operate in the centre of its resonance band, implying that the stack's capacitance and the transformer's inductance is matched at the resonant frequency. The basic problem is that the stack's capacitance can markedly change due to changes in either temperature or externally applied pressure, or both. These effects can push the tank circuit out of resonance, leading to inefficient use of the power source. The stack must necessarily be subject to the mechanical compression and tension of drillstring forces transferred into the mandrel and cover, primarily because it must transfer its wave energy out into the drillstring via the mandrel and cover. The dynamic mechanical loading of the stack due to varying drilling conditions is particularly difficult to manage, and ideally would require a closed loop system to compensate. Temperature changes, although not so changeable as pressure, are still significant and thus also have a significant effect on the stack.